Energy efficiency optimization for fading MIMO non-orthogonal multiple access systems

Non-orthogonal multiple access (NOMA) is expected to be a promising multiple access techniques for 5G networks due to its superior spectral efficiency (SE). In this paper, we study the energy efficiency (EE) optimization for the fading multiple-input multiple-output (MIMO) NOMA systems with statistical channel state information (CSI) at the transmitter. The EE optimization problem is formulated to maximize the system EE (defined by ergodic capacity under unit power consumption) under the total transmit power constraint and the minimum rate constraint of the weak user. We propose the near optimal power allocation schemes and also the suboptimal closed-form solutions. Numerical results show that the proposed NOMA schemes significantly outperform the traditional orthogonal multiple access scheme with open loop MIMO transmission in terms of both SE and EE.